1. Field of the Invention
The present invention relates generally to computer-graphics apparatus and more particularly to a computer-graphics display system for displaying graphic information and enabling various operations to be performed upon the displayed information with minimal participation from the host computer once the raw data has been transferred from the host computer into the display system.
2. Description of the Prior Art
Among the various systems heretofore used to display computer-graphics information are the following:
Random stroke refresh display systems--In these types of devices an instruction list describing the graphics picture as lines and arcs, etc., is kept in a display memory and the entire list is read and transformed from list coordinates to screen coordinates by ultra high speed logic. Each line or arc is then "painted" on a display screen by deflecting an electron beam directly along the line coordinates and the entire list is typically periodically repainted at a rate of between 40 and 60 times a second. Selective erase or change of displayed information is accomplished by editing the picture list. These displays are often capable of zoom and pan operations accomplished by means of transformation hardware. The major limitations of this technique have been expense and allowable picture complexity, the latter referring to a practical limit as to how long a picture list can be before consequent flicker of the display makes it unusable by a human operator.
Direct view storage tube systems--In apparatus of this type an electron beam paints a picture directly on a bystable phosphor-coated screen, which then stores the image until a high voltage erase pulse floods the screen to return all the phosphors to the unwritten state. The picture can be of very high complexity, good quality curved lines can be generated and display flicker is not a problem. This technique has been preferred over the past few years for low cost graphics system. A disadvantage of such apparatus is that no pan or zoom of stored image can be accomplished and no selective erase of stored phosphors is permitted. Moreover, the phosphorous storage tubes have two further limitations in that they characteristically have low luminesence requiring subdued room lighting for best utilization, and the tubes normally age, especially around the center and edges of the display screen, and typically require replacement once or twice a year. Tube replacement is a high cost item which over a three-year period can cost as much as 80% to 200% of the initial purchase price of the display apparatus.
Plasma panel systems--A plasma panel is comprised of small neon gas discharge tubes arranged most popularly in a 512.times.512 matrix and provides a much brighter picture than the previously mentioned display tube. However, systems incorporating such panels cannot zoom or pan the stored image. With the exception that limited selective erase is permitted, the plasma panel display is similar to the storage tube display in that each neon tube "remembers" its on/off state and no complexity limit or flicker is apparent. Although the 512.times.512 raster causes some graininess in curved lines, the most serious drawback of this type of display for graphics uses is that no method of implementing a cursor (targeting symbol) on the panel is offered, whereas all other prior art devices provide such a feature.
Scan conversion memory systems--This technique uses an indirect view storage tube wherein a picture is drawn on a semiconducting surface with an electric charge. A reading beam is then swept over the charged surface in a raster pattern and the beam readout is output to a TV monitor. A major use of the scan conversion technique has been to convert European standard TV signals (over 600 lines) to American standard TV signals (525 lines). The display device operates much like a direct view storage tube and is capable of displaying a picture of high complexity. Good quality curved lines can be generated and various shades of grey can be displayed. At least two graphics devices of this type have been introduced since 1973 with both devices using interlaced video at 60 fields/30 frames per second. Zoom and pan are possible but of limited value since the effective resolution of the scan converter seems to be of about a 300 dot square, much too coarse to justify much zooming. By way of comparison, the direct view storage tube devices seem to have about two to four times the resolution of these types of devices. Limited selective erase is permitted on scan conversion displays and a video cursor may be mixed in with the video but with a 3%-5% estimated positional error since the cursor is not written on the storage surface and many variables such as beam focusing, intensity deflection, and pin cushion errors sum together to effect cursor misalignment. Under zoom, any cursor position error is even further exaggerated. Horizontal line flicker, an effect known as the "Kell factor" is also inherent in these types of displays.
Serial raster displays--These devices use a serial digital memory implemented from shift registers (using integrated circuit, CCD, magnetic bubbles or other techniques) or rotating serial memories, i.e., magnetic disks, or drums, or other rotating devices. The video control units utilized in such systems are relatively simple and no devices currently on the market include pan, zoom, or split-screen features. Although the displayed picture can be of very high complexity, the cost of such devices is slightly higher than the storage tube devices. The typical dot matrix for such systems is a single 256.times.256 raster with an alternate 512.times.512 raster as an extra cost option. In present systems, limited selective erase is offered with no XOR capability. Color display options are also offered but increase the price of the system by a factor of two or three. Good graphic cursors may be provided with essentially no location error between cursor and picture. The limitations of such devices are slow dot writing speeds, because of limited access to individual bits in the serial memory, and limited resolution causing very distinct graininess in curved lines. No such system offers split screen, zoom, pan, or XOR.
Random access raster displays--These types of systems are generally similar to the serial raster displays mentioned above but employ random access digital memories, (magnetic cores, integrated circuits, etc.) for the raster memory. Several devices of this type have recently been introduced, mainly due to the reduction in the cost of random access memories. Typical formats mentioned are 256.times.256 bits with 512.times.512 and color offered as optional extra cost features. The principal advantage of these devices over the serial type devices is faster dot write and erase time. Other performance characteristics are substantially identical to the serial raster displays and no system on the market offers split screen, zoom, pan or XOR.
U.S. patents relating to the above types of display systems include Strout, No. 3,396,377; Okuda et al, No. 3,836,902; and Schwartz et al, No. 3,906,480.